This invention relates to a loop antenna, and more particularly to a square loop directive antenna.
A loop antenna is an antenna which provides an actual or virtual closed circuit through one or more turns of a conductor. In the case of a "large" loop, which is defined as one in which the current is not the same either in amplitude or phase in every part of the loop, the smallest length of conductor which can be used is 1/2 wavelength. The practice is to form the loop into a square 1/8 wavelength on each side, and to feed it at the center of one side. The current flows in the closed loop with maximum current at the center opposite the drive terminals, and minimum at the terminals. The result of such an antenna configuration and drive causes the field radiated to be maximum in the plane of the loop and in a direction from the drive terminals to the opposite side, unless the opposite side is opened at the center, in which case there is only a virtual closed circuit and the direction of maximum radiation is reversed. Such an open square loop antenna may be compared to a half-wavelength dipole antenna because it is in effect a dipole antenna that has been folded into a square loop with the ends of the dipole next to each other on the side of the loop opposite the drive terminals. However, unlike a half-wavelength dipole, there is no direction in which the radiation is zero. While maximum radiation is in the plane of the loop, there is appreciable radiation in the direction perpendicular to that plane, and there is also appreciable radiation to the "back," the direction opposite the "front" where the radiation is a maximum. The front-to-back ratio is of the order of about 5 db, and the front radiation is about 1 db less than that of a half-wavelength dipole in its optimum direction.
That ratio of front radiation to the back radiation of a square-loop antenna can be enhanced, and the gain can be increased to 1 db over a dipole antenna, by placing inductive reactances in the sides joining the front and back to the loop. This is because the inductive reactances decrease the current in the sides where they are included and increase it in the front. It has now been discovered that the same improvement can be achieved with capacitive reactance instead of inductive reactance, with only 180.degree. change in the direction of maximum radiation.